The use of electro-optical materials to switch polarized laser beams between an on condition and an off condition is well known. Electro-optical materials are useful for this purpose because they are capable of changing the polarization of a light beam in response to the application of an electrical voltage across the material. Depending on the nature of the electro-optical material, its size, and the amount of voltage which is applied to the material, the polarization of a light beam can be varied to selectively pass a polarizer which has a predetermined polarization orientation.
When used in a regenerative amplifier, an electro-optical material is commonly referred to as a Pockels' Cell and functions generally as follows. Initially, the Pockels' Cell is nominally in an OFF condition. While the Pockels' Cell is in the OFF condition, a laser medium in the regenerative amplifier cavity of the apparatus is "pumped" to generate an excess of excited atoms in the medium. After the medium has been "pumped" the Pockels' Cell is then switched to an ON condition to capture an unamplified ultra-short laser pulse in the cavity. While in the cavity, the captured pulse is reflected back and forth through the laser medium to amplify the pulse. This amplification process takes approximately 300 nanoseconds, after which, the Pockels' Cell is again switched to an OFF condition to "dump" the amplified pulse from the cavity.
To effectively perform the function of a Pockels' Cell, the polarization of a laser beam which passes the electro-optical material needs to be selectively changed from the OFF condition to the ON condition, and from the ON condition back to an OFF condition. These changes in condition must be accomplished within a time span of approximately 4 or 5 nanoseconds in response to the selective application of an electrical voltage across the material. Preferably, at least for a regenerative amplifier, the voltage will alter the characteristics of the electro-optic material so that it changes a linearly polarized light to a circularly polarized light which is frequently referred to as quarter wave polarization shift. With such alterations, polarized laser light which was previously unable to pass freely through the polarizer (i.e. an OFF condition) can now do so (i.e. an ON condition), and vice versa.
Typically, in order to alter the polarization of the laser light using a Pockels' Cell, a relatively large voltage is required to be switched between the two electrodes of the electro-optic crystal. Also, once a voltage is applied, it is not a simple matter to remove the voltage from the material and thereby restore the characteristics of the Pockels' Cell to the state or condition it was in before application of the voltage.
Heretofore, due to the capacitive nature of a Pockels' Cell, and the requirement for very rapid changes in the polarization of the laser beam, it has been easier to apply successive voltages to the Pockels' Cell rather than apply and then remove a voltage. Thus, to switch a Pockels' Cell through an OFF/ON/OFF cycle, a quarter wavelength voltage was first applied to the electro-optical material to change the Pockels' Cell from OFF to ON. A double quarter wavelength voltage was then subsequently applied to the electro-optical material to change the Pockels' Cell from ON to OFF. The double voltage was then allowed to dissipate in order to return the Pockels' Cell to its pre-cycle state. Unfortunately, this double voltage causes greater acoustical disturbance for the Pockels' Cell, requires more power and, consequently, generates more heat. Moreover, during dissipation of the voltage, the Pockels' Cell transitions the ON condition for a second time, and this allows partially amplified light to leak from the pulsed laser beam generating apparatus. Such leakage disturbs the pulsed laser beam and diminishes the efficacy of the apparatus for its intended purpose. Furthermore, depending on the particular application, the leakage can cause injury.
The present invention recognizes that a switch driver system for an electro-optical material can be provided which effectively applies and removes a voltage from the material to allow for the generation of sharp changes in the polarization of the passing laser light. Further, the present invention recognizes that such a switch driver system can effectively operate between only two conditions. One condition being wherein a voltage is applied to the electro-optical material to establish a first optical transmission characteristic for the material, and the other condition being wherein the voltage is removed from the electro-optical material to establish a second optical transmission characteristic for the material.
In light of the above, it is an object of the present invention to provide a two-step optical switch driver system which helps minimize the acoustical disturbance of the electro-optical material. Another object of the present invention is to provide a two-step optical switch driver system which has reduced power requirements and, thus, generates less heat in the system. Still another object of the present invention is to provide a two-step optical switch driver system which avoids transmission of an ON condition wherein unwanted laser light leakage can occur. Another object of the present invention is to provide a two-step optical switch driver system which has a very sharp turn on and turn off capability in order to capture very short duration laser pulses from a laser pulse train having a very high repetition rate. Yet another object of the present invention is to provide a two-step optical switch which is relatively simple to manufacture, easy to use and comparatively cost effective.